1. Field of the Invention
This invention relates to a furnace for growing a compound semiconductor single crystal and a method of growing the compound semiconductor single crystal by using the furnace. Especially, the furnace is suitably used for growing a compound semiconductor single crystal with a zinc blende structure such as gallium arsenide (herein referred to as GaAs), indium phosphide (herein referred to as InP) and gallium phosphide (herein referred to as GaP).
2. Description of the Related Art
One of methods for growing compound semiconductor single crystal such as GaAs and InP is a vertical boat method that a material melt is charged in a growth furnace (crucible), the crystallization starts from a seed crystal previously placed at the bottom of the growth furnace, the crystallization proceeds gradually to upward, and finally the whole material melt is crystallized. The vertical boat method includes a vertical Bridgman (VB) method that a crystal is grown by moving a crucible from a high temperature section to a low temperature section in a hot zone with an adjusted temperature profile, and a vertical gradient freeze (VGF) method that a crystal is grown by gradually lowering the furnace temperature while the temperature gradient remains stable. It is generally known that the vertical boat method can offer a compound semiconductor single crystal with reduced crystal defect such as a dislocation since the crystal growth is conducted in a temperature gradient smaller than the Czochralski pull method.
FIG. 1A is a vertical cross sectional view showing a conventional single crystal growth furnace 101 used for the vertical boat method. FIG. 1B is a cross sectional view cut along a line A-A′ in FIG. 1A.
The crystal growth furnace 101 is generally composed of: a small diameter section 100a that is at the bottom formed like a well and serves as a seed crystal charge region; an increasing diameter section 101b that is formed like an inverted truncated cone tube and has diameters increased from the small diameter section 101a to upward; and a constant diameter section 101c that is formed cylindrical and has a nearly constant diameter and serves as a crystal growth region lying from the increasing diameter section 101b to upward.
It is known that there is a close relationship between the an angle (a) of inner wall in the increasing diameter section 101b and a probability of twin crystal generation when a zinc blende structure compound semiconductor single crystal such as gallium arsenide is grown using such a crystal growth furnace by the vertical boat method.
It is known that a GaAs single crystal etc. is grown in the (100) direction and, therefore, a (111) facet is generated in the increasing diameter section 101b and a twin crystal is generated from the facet. Since an angle formed between the (100) plane and the (111) plane is 54.7 degrees, the angle (a) of inner wall in the increasing diameter section 101b as shown in FIG. 1A is generally set to be 70.6 degrees (=180 degrees−54.7 degrees×2) or less. However, when the angle (a) of inner wall in the increasing diameter section 101b decreases, the single crystal obtained must have a long crystal portion corresponding to the increasing diameter section 101b. Thereby, the yield of wafer lowers and the production efficiency lowers. Because of this, it is tried that the angle (a) of inner wall in the increasing diameter section 101b is set to be about 80 degrees to 100 degrees. However, in terms of the prevention of twin crystal generation, a sufficient effect is not obtained.
Japanese patent application laid-open No. 5-194073 (prior art 1) discloses a crystal growth method that a crucible used has 160 degrees to 200 degrees in the angle (a) of inner wall in the increasing diameter section, its crystal is grown in a nearly horizontal direction while making locally a region near the seed crystal into a supercooled state, the crystal growth is further conducted such that a boundary surface of solid phase and liquid phase (herein referred to as solid-liquid interface) is convex toward the melt side, and thereafter its material melt is solidified by cooling in a temperature gradient of 5° C./cm to 15° C./cm.
Japanese patent application laid-open No. 10-87392 (prior art 2) discloses a crystal growth method that a crucible used has 160 degrees to 180 degrees in the angle (a) of inner wall in the increasing diameter section, and the temperature gradient in the growth direction of at least the increasing diameter section is controlled to be 1° C./cm to 5° C./cm during the crystallization.
However, it is noted by the inventors that the crystal growth methods of prior arts 1, 2 have problems as described below.
In prior art 1, in order to control the temperature profile to make the solid-liquid interface convex toward the melt side, it is required that the temperature gradient is 5° C./cm or more during the cooling. However, when the temperature gradient is 5° C./cm or more, it is difficult to sufficiently prevent the generation of twin crystal or polycrystal because a large fluctuation in temperature is caused by a convection in the material melt. Further, its heat sink needs to have a cooling medium in order to control the temperature profile. Therefore, the manufacturing cost increases.
In prior art 2, it is required that the crucible used has 160 degrees to 180 degrees in the angle (a) of inner wall in the increasing diameter section, and the temperature gradient in the growth direction of at least the increasing diameter section is controlled to be 1° C./cm to 5° C./cm during the crystallization. However, in the case of controlling to be 160 degrees to 180 degrees in the angle (a) of inner wall in the increasing diameter section and to be 1° C./cm to 5° C./cm in the temperature gradient in the growth direction, it is difficult to control the supercooled state of material melt in the shoulder section. Thus, since the growth of (111) facet is promoted, it is difficult to sufficiently prevent the generation of twin crystals. Furthermore, since the temperature gradient is small and the shoulder angle is large, crystal nuclei may be generated from other than the seed crystal section to have polycrystal.